One technique to measure formation resistivity involves the use of electromagnetic induction via transmitters of low frequency magnetic fields that induce electrical currents in the formation. These induced electrical currents in turn produce secondary magnetic fields that can be measured by a magnetic field receiver.
Resistivity measurements of the earth's subsurface between proximately located wellbores are known as “crosswell measurements”. Various tools and methods to perform crosswell logging, and thereby obtain crosswell measurement data sets, are known in the art. A crosswell data set is typically collected by moving a transmitter in one well and a receiver array in another well. Measurements of the electric and magnetic fields corresponding to various positions of the transmitter and receiver array are made. A sophisticated inversion process is necessary to interpret this data set and obtain a resistivity image of the region between the wells. A typical crosswell logging system uses a transmitter that produces a large vertical magnetic moment and very sensitive receivers that are particularly sensitive to the vertical moment of the magnetic field.
Several of the systems designed for crosswell logging use magnetic field transmitters and receivers. Those transmitters and receivers produce magnetic dipole moments oriented along the longitudinal axis of the tool. In principal, either or both of the wells can be cased. However, in practice, the use of standard magnetic steel casing for both wells is problematic due to the severe attenuation caused by the casing. Successful and useful logs have been obtained where both wells are open holes or where the receiver well is cased with magnetic casing. In addition, successful logs have been obtained where one or both wells are cased with steel having a high chromium content. That casing is non-magnetic and has a lower conductivity than standard casing. As a result, the attenuation is much less than with standard magnetic steel casing.
There are also various tools that transmit an electromagnetic signal from within a wellbore to one or more receivers located on the surface. For example, there is a wireline tool designed to measure formation resistivity in cased wells, a logging-while-drilling (LWD) tool that sends a current across an insulated gap in the tool and into the formation, a production logging tool that provides for wireless telemetry between a downhole component and the surface, and an EM telemetry tool used as an alternative to mud pulse telemetry that applies a voltage across an insulated gap in the tool to create an E-dipole source (E-Pulse is described in commonly owned/assigned U.S. Pat. No. 7,477,162).